The present invention generally relates to a coating composition and to a method of using the coating composition. More particularly, the present invention relates to a coating composition capable of bonding to both oxidizable metal and a polymeric coating and to a method of using the coating composition during manufacture of small electronic devices.
Small electronic devices based on lead frames carry microelectronic components, such as an integrated circuit (IC) chip, typically have exposed metal portions that are subject to oxidation and, thus, corrosion. Lead frames, for example may be constructed of copper; aluminum; nickel; a noble metal, or various ferrous alloys that are all potentially oxidizable to some degree. After manufacture, lead frames are typically treated with an anticorrosion coating to inhibit corrosion prior to attachment of components to the lead frame. Adhesive bonding of microelectronic components, such as a microchip made of semiconductor material, to the lead frame may complicate the corrosion issue. Also, the industry practice of encapsulating at least a portion of the small electronic device in plastic can further complicate the corrosion issue.
Various attempts have been made to supply anticorrosion coatings that may be applied to exposed metal portions of small electronic devices to inhibit or eliminate corrosion. While some of these anticorrosion coatings may offer adequate corrosion protection, at least in some circumstances, adhesion of bonding materials to the underlying lead frame and adhesion of the encapsulating plastic coating, sometimes referred to as “packaging” the small electrical component, remains problematic.
Some anticorrosion coatings are so problematic to adhesion of bonding materials and encapsulating plastic coatings that removal of these anticorrosion coatings is typically undertaken prior to applying the bonding material or the encapsulating plastic coating. This solution is unsatisfactory for a number of reasons. First, this solution adds an additional step to the manufacture of small electronic components that requires both additional resources and time. Furthermore, it can be difficult, if not impossible, to remove all anticorrosion coating material from all coated surfaces. Difficulties can arise from the geometries of surfaces coated with anticorrosion coatings. Also, non-metallic surfaces (ceramic, for example) can exist in small electronic devices. It can be challenging, or at least impractical, to avoid application of anticorrosion coatings to these non-metallic surfaces. Removal of some anticorrosion coatings from non-metallic surfaces can be difficult, if not impossible, due to surface geometries of some coated non-metallic surfaces. Furthermore, use of etching chemicals may be required in some cases to effect removal of anticorrosion coatings. Practical difficulties in the course of applying these etching chemicals can undesirably alter properties of a component of the small electronic device.
Not all anticorrosion coatings are necessarily problematic to adhesion of bonding materials or encapsulating plastic coatings. However, for these anticorrosion coatings, another problem often arises. In particular, when electronic components are being thermally bonded in the course of manufacturing the small electronic components, the heat generated by the thermal bonding often causes anticorrosion coatings to either delaminate or desorb from the coated surface or even chemically decompose. Any delaminated or desorbed portions or residues from decomposed anticorrosion coating must be cleaned and recoated with anticorrosion coating before applying the encapsulating plastic coating, which again undesirably requires additional resources and adds time to the manufacturing process.
An alternative approach that has been tried, where existence of the anticorrosion coating is problematic to adhesion of the encapsulating plastic coating, entails applying an adhesion-improving coating over the anticorrosion coating. This approach, too, has generally not been satisfactory. The adhesion-improving coating is generally applied prior to diebonding of the semiconductor material that is etched to form a micro chip and conductive vias. Therefore, complete coverage of adhesion-improving coating on conductive areas within process windows is typically not realized. Another attempted solution entails electrolytic application of an adhesion-promoting coating after incorporation of the chip in the small electronic device. This approach is also unsatisfactory, since the electrolytically applied adhesion-promoting coating only attaches to conductive surfaces and leaves various substrate surfaces (i.e. ceramic, plastic) uncoated.
The various existing attempts to solve and adequately address the corrosion prevention issue and improved adhesion of bonding materials and encapsulating plastic coatings have increased the knowledge base pertinent to both issues. Nonetheless, a need still exists for a solution that both prevents corrosion of metal surfaces that are susceptible to corrosion while supporting firm attachment of bonding materials and encapsulating plastic coatings.
For these and other reasons there is a need for the present invention.